1. Field of the Invention
The invention relates to a photo-mask for a stepper, and more particularly to a five inch photo mask, on which a set of four 15 mm.times.15 mm alignment marks of and a set of four 20 mm.times.20 mm alignment marks are both tooled, suitably used for a NIKON stepper for mix-match production of G-line and an I-line.
2. Description of the Related Art
Integration circuits (ICs) have reached every part of our daily lives. However, the process of fabricating an IC is very complex. Hundreds of steps are needed for making an IC. The fabrication normally takes one or two month to complete. The IC industry is a high technology industry which includes four main branches: IC design, wafer fabrication, wafer testing, and wafer packaging. Thus, the IC industry is not only an advanced technology industry, but also a high risk industry which needs tremendous capital to maintain.
For wafer fabrication, processes such as oxidation, diffusion, deposition, patterning, and etching, are further included. While exposing the photo-resist layer on the surface for patterning a wafer, in addition to a light source, a photo mask for transferring the pattern is necessary.
The main body of a photo mask is a flat and transparent glass plate. Patterns of semiconductors are indicated by a chrome border having a thickness of hundreds of angstroms, coated on the glass. Sometimes, a chromium oxide layer of thickness 200 .ANG. is further formed on the chrome border for the purpose of anti-reflection. For a normal process of forming a device, 10 to 18 photo masks are used. In every step of exposure, an alignment is necessary to obtain a precise pattern. Otherwise, a device failure is caused due to improper pattern transfer.
There are three conventional techniques for exposing photo-resist, including technique respectively of a contact type, a proximity type, and a projection type. For the projection type exposure technique, the characteristic most different from those of the former two types is that the photo mask does not contact, nor is it proximate with the device. The advantage of this characteristic is that the pattern on the photo mask is not damaged by contact during pattern transfer, and good resolution can be achieved. A more advanced projection-type exposure technique is the "step and repeat" exposure, whereby the resolution is further enhanced. The equipment used for this technique is called a "stepper".
In a photolithography process used for commercial production, a stepper is used to transfer a pattern. However, the stepper is not without problems. The main problem is that, for commercial mass production, tens of exposure processes are required. Thus, the yield is reduced as the fabricating time increases. The more frequently used light sources are a G line source and an I-line source. The G line source (G4/G61G7) is a source of ultra-violet light having a wavelength of 4360 .ANG., and the I line source (I8/I9) is a source of ultra-violet light having a wavelength of 365 .ANG..
Referring to FIG. 1, a layout of a photo mask with four 15 mm.times.15 mm alignment marks is shown. The photo mask comprises a 5 inch-square reticle glass plate 10 and a chrome border 20 coated on the reticle glass plate 10. The pattern to be transferred is indicated by the chrome border 20. The four alignment marks, illustrated as a first alignment mark 25, a second alignment mark 30, a third alignment mark 35, and a fourth alignment mark 40 are tooled along the respective four sides of the chrome border 20. A thin pellicle frame 15 is coated on the chrome border 20 to protect the surface of the photo mask from being contaminated by particles or granules. The dimension of the pellicle frame 15 is 120 mm.times.98 mm.
Applying a conventional photo-mask with a set of four 15 mm.times.15 mm alignment marks to an 18 or 19 stepper for exposure, an extra machine test is needed to ensure the accuracy of the process. The extra machine test takes about 2 hours per set to complete. In case that the 18 or 19 stepper is abnormal, after resetting the stepper, without performing the extra machine test, a device failure may occur. A normal mass production yield can not be obtained, and a flexibility of production is poor.